Sulfone polymers are high performance amorphous thermoplastic engineering resins that contain the characteristic diaryl sulfone linkage. Sulfone polymers are known for their high mechanical strength, thermal and oxidative resistance, resistance to hydrolysis, and to many acids, bases, and solvents.
Polysulfone is a well-known high temperature amorphous engineering thermoplastic resin. It exhibits a high glass transition temperature of about 185° C., high strength, stiffness and toughness over a temperature range from about −100 to 150° C. Being completely amorphous, the polymer also exhibits transparency, which adds to its utility in many end uses. Polysulfone was commercially introduced in 1965 by the Union Carbide Corporation. It has the chemical structure:

The polysulfone shown above, commonly abbreviated as PSU, is probably the most commercially important member of a broad family of aromatic backbone polymers known as polyarylethers. These polymers can be produced by a variety of methods. For example U.S. Pat. Nos. 4,108,837 and 4,175,175 describe the preparation of polyarylethers and in particular polyarylethersulfones. Several one-step and two-step processes are described in these patents, which patents are incorporated herein by reference in their entireties. In these processes, a double alkali metal salt of a dihydric phenol is reacted with a dihalobenzenoid compound in the presence of sulfone or sulfoxide solvents under substantially anhydrous conditions. In a two-step process, a dihydric phenol is first converted, in situ, in the presence of a sulfone or sulfoxide solvent to the alkali metal salt derivative by reaction with an alkali metal or an alkali metal compound. In the case of PSU manufacture, the starting monomers are bisphenol A and a 4,4′-dihalodiphenylsulfone, typically 4,4′-dichlorodiphenylsulfone. The bisphenol A is first converted to the dialkali metal salt derivative by first reacting with a base like sodium hydroxide, NaOH, in a 1:2 stoichiometric molar ratio to produce the disodium salt of bisphenol A. This disodium salt of bisphenol A is then reacted with 4,4′-dichlorodiphenylsulfone in a second step to produce the polymer. Sodium chloride salt is produced as a byproduct of the polymerization.

The salt is filtered, then the polymer solution is either contacted with a non-solvent to precipitate the polymer or, alternatively, the polymer is recovered by evaporative removal of the solvent. In either case, the solvent removal is usually followed by forming of the polymer into pellets in an extruder, preferably a twin screw extruder.
Among the many desirable physical characteristics and attributes of PSU, this polymer is transparent in its natural state. The transparency of polysulfone is useful in combination with its high heat and other high performance attributes. Examples of uses where the transparency is useful include covers and lids for hot serving dishes and containers, lids for medical sterilization trays, research lab animal cages, dairy processing equipment, flow meters and sight glasses for chemical process equipment.
The transparency of PSU, coupled with the high refractive index of the material relative to other transparent thermoplastics (1.63 versus 1.59 for polycarbonate), make polysulfone a candidate for use in lens applications as it can allow the design of higher powered lenses for a given lens thickness and weight, or alternatively design thinner and lighter lenses relative to polycarbonate for a given power or diopter rating. The high refractive index allows lens makers to produce high-powered lenses with relatively low curvature (and hence lower mass) relative to what is possible with lower index materials such as glass, polymethylmethacrylate (PMMA) and conventional thermosetting plastics used for this purpose. Because of this feature, polysulfone is therefore particularly attractive for use in ophthalmic lenses for spectacles used in prescription eyewear.
In the ophthalmic lens industry a material is considered ‘high index’ if its refractive index is 1.60 or higher. As such, polysulfone is poised to become the first thermoplastic resin in the high index category. However, polysulfone's entry into the ophthalmic lens industry, and indeed into most other optical applications, has been hampered by the yellowness that, until now, has been present in all commercially available polysulfone to date. Apart from being aesthetically undesirable, the yellow cast also limits the light transmittance that is critical for a high clarity lens for prescription eyewear. Water white or near water white clarity is a key requirement for any lens material, and, to date, the state of the art of polysulfone manufacture has not yet allowed the production of resin with the type of clarity that is needed. Polysulfone has been of interest to the ophthalmic lens industry for a long time as it offers a number of attractive features. In addition to providing a high refractive index polysulfone offers low cost thermoplastic lens fabrication methods (i.e. hybrid injection-compression molding). In addition to the reduced lens thickness and weight which are desirable to the consumer, the good impact resistance of polysulfone allows thin lenses to be viable.
For an ophthalmic lens material to be viable, it is generally accepted that it must meet the following three optical property requirements:                1. A low yellowness index, as commonly measured by ASTM method D-1925, is needed. Yellowness index is a thickness dependent property. Yellowness index values below 1.0 are generally desirable, but at minimum, the material must have a yellowness index of 2.0 or less at a thickness of 0.1 inch (2.5 mm). Yellowness indices below 2.0 are difficult to discern with the naked eye and may be considered of sufficient quality for optical lens uses in general and ophthalmic lenses in particular.        2. A high light transmittance as commonly measured by ASTM method D-1003 is also a key requirement. Light transmittance values greater than 85% are needed as a minimum. Light transmittance is also a thickness dependent property although generally to a lesser extent than yellowness index. It is commonly measured at a thickness of 0.1 inch (2.5 mm), so that if the transmittance requirements are met at 0.1 inch (2.5 mm) thickness, they will be automatically met at reduced thicknesses.        3. A low haze as measured by ASTM method D-1003 is also a requirement. Haze is the ratio of the diffuse light transmittance to the total light transmittance through a specimen expressed as a percent. It generally needs to be below 2.0% and preferably below 1.0% for 0.1 inch (2.5 mm) for a high clarity or optical quality material. Haze values below 2.0% are difficult to discern by the naked eye and thus are acceptable. Like yellowness index and light transmittance, haze is also dependent on specimen thickness, so it is important to compare haze between different materials only at comparable thicknesses and specimen surface characteristics.        
Historically, Union Carbide, Amoco, and then Solvay Advanced Polymers, LLC have measured and tracked the color of all sulfone polymers using the internal parameter of color factor (CF). The plastics industry as a whole, on the other hand, uses yellowness index (YI) to quantify color of film and moldings. It is instructive to look first at these two quantities and how they relate to each other.
Yellowness index and color factor are two different quantities from the standpoint of the definition of the parameter. However, for practical purposes, they do correlate very well.
By definition, yellowness index (YI) is calculated from the equation below based on ASTM method D-1925:YI=[100(1.28X−1.06Z)]/Y where in the equation above, X, Y and Z are the tristimulus transmittance components for red, green and blue lights, respectively, in the CIE system, based on illuminating the sample with a standard light source, such as illuminant C or illuminant D65 according to ASTM method D-1003.
Color Factor (CF), on the other hand, is defined as the following quantity:CF=270[(x+y)sample−(x+y)air]/t Where x and y are the chromaticity coordinates obtained by normalizing the X and Y tristimulus values. The chromaticity coordinates x and y are calculated by the following equations:x=X/(X+Y+Z)y=Y/(X+Y+Z)The variable t is the sample thickness in inches. So, unlike YI, CF is independent of thickness in the thickness range of typical molded components, which is one attractive aspect of the quantity. Color factor is independent of thickness up to about 1 inch thick. The 270 factor is an arbitrarily chosen factor intended primarily to bring the CF values into a convenient range to work with.
As mentioned above, yellowness index, light transmittance and haze are all thickness dependent properties so that thickness needs to be reported along with these measurements. Preferably multiple thicknesses should be measured to show the dependence of these properties on thickness over a practical range of thicknesses.
One of the key technical hurdles to achieving that goal is the elimination of yellowness from the resin, which is typically expressed as a color factor. A color factor target of <10 has been set for plastic molded parts to be considered of optical quality. This corresponds, in yellowness index terms (ASTM D-1925) to yellowness index of <1.9 for a 0.1 inch thick sample plaque. The lowest color factors achieved in molded articles of prior art commercially produced polysulfone have been in the 30-40 color factor range and more typically they have been in the 50-70 range. While PSU with solution batch color factors of under 25 has been produced based on improvements in PSU manufacture technology as described for, example, by Schwab et al. in U.S. Pat. No. 4,307,222, which is incorporated herein by reference in its entirety, the ability to produce melt fabricated articles with color factors of less than 25 has not been demonstrated.
To achieve the single-digit color factors needed, technology improvements are necessary in either or both the synthesis-process side and in the stabilization of the pellets supplied to the customers for prevention of additional color generation during melt fabrication into injection molded articles.
While lab batches of polysulfone with color factors under 10 CF can be produced, it is difficult to maintain this low color factor even with the mildest melt processing treatment. This behavior is exemplified by the plot shown in FIG. 1. This plot shows the progression of color factor of two polysulfone batches. Samples of the polysulfone powder were heat aged in a melt indexer at 300° C. and various times to monitor the dependence of color factor on 300° C. exposure time. It can be seen from FIG. 1 that even after 2 minutes at 300° C., both polysulfone samples approximately double in color factor, and after 12 minutes the color factor has risen to roughly 3× the original value. This behavior is disconcerting, considering that 300° C. is at the lower limit of where polysulfone can realistically be melt fabricated by injection molding.
FIG. 2 is a graph showing the correspondence between CF and YI for a number of UDEL® lots as measured on plaques 0.1 inch thick. As can be seen from this figure, the relationship between the two variables is essentially a straight line that passes through the origin. For a given color factor, the corresponding yellowness index on a 0.1 inch thickness specimen is closely approximated by multiplying CF by 0.19. The straight line relationship between CF and YI shows how the color factor measurement relates to the more widely used yellowness index parameter.
Yellowness in polysulfone has been believed to be primarily responsible for most of the absorbance over the visible spectrum. Thus, achieving the high transmittance characteristics exhibited by a colorless resin like polycarbonate had been largely equated with removal of yellowness. To assess this hypothesis, polysulfone with different color factors was correlated with the transmittance characteristics at various wavelengths. A family of curves illustrating transmittance dependence on color factor is shown in FIG. 3. At incident wavelengths above 540 nm, the transmittance is essentially independent of UDEL® color factor within the color factor range of interest (0 to 60). However, the dependence becomes progressively stronger at shorter and shorter wavelengths and is rather steep at wavelengths in the 400-420 nm range. Since recent experiments have produced polysulfone plaques with color factors in the low twenties, these new low color samples were used in combination with other data to allow extrapolation and prediction of transmittance behavior in the 0-10 target color factor range.
The production of <10 color factor UDEL® polysulfone in lab glassware is feasible, however, it has not previously been possible to maintain that low color factor through even the most mild melt processing. It was therefore concluded that a color stabilization package for polysulfone would be necessary if a viable ultra-low color polysulfone that can maintain its color during injection molding into finished optical elements is to be produced.
To develop an ultra-low color/optical quality polysulfone resin, a series of experiments was conducted to screen and optimize an appropriate additive package that would prevent or minimize color development during melt fabrication of the polymer. In studies conducted it was established that color factors under 10 could be achieved for polysulfone as made in the reactor. The color, however, rapidly rises to unacceptable levels when the recovered polymer is exposed to temperatures as low as 300° C. for times as short as 2 minutes. It became clear that a resin stabilization scheme was necessary and a key part of the solution to the polysulfone color problem.
From the extrapolations shown in FIG. 3, hypothetical spectral transmittance curves were generated for UDEL® polysulfone having 10 and 0 color factors to see how they contrast against the transmittance curve of general purpose polycarbonate resin (LEXAN® 104 available from General Electric). This spectral transmittance curve comparison is shown in FIG. 4.